Process for the extraction of solid material from solution by crystallization

ABSTRACT

A method of obtaining crystals of a solute in which the fresh solution is initially cooled by contacting with a mother liquor containing crystals below the desired grain size to produce a mother phase in a state of incipient crystallization. The latter is crystallized with or without further cooling by passing it through an elongated crystallizer body with angularly adjoining portions swung around an offset axis and having cooled walls. The resulting slurry is then continuously filtered to separate out the crystals of the required size and form the mother liquor containing particles below the required size.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 131,826 filed Mar. 19, 1980,now U.S. Pat. No. 4,357,306 issued Nov. 2, 1982.

FIELD OF THE INVENTION

The invention relates to a process for the extraction of solids fromsolution by crystallization.

BACKGROUND OF THE INVENTION

In the chemical-, pharmaceutical industry and in other industrialsectors it is an essential task to produce crystalline, solid materialfrom solution, which is easily treatable in the course of subsequentprocesses. Several crystallizers are known, functioning with cooling,distillation, evaporation cooling or pulverization.

A cooling crystallizer with grading screw is described by: Dr. MucskaiL. "Kristalyositas" ("Crystallization"), Muszaki Konyvkiado, Budapest,1971, page 151. This equipment has a mother liquor tank, cooler andgrading screw at an angle above the mother liquor tank. The warm, freshsolution passes into the mother liquor tank where it is mixed with themother liquor arriving on overflow from the grading screw. The crystalslurry after passing through the cooler enters the grading screw, whichhas a sloping, trough-shaped bottom, on the lower part of which thelarger crystals settle. These are removed from the trough by the screw.The mother liquor overflowing from the grader--together with the smallercrystals--flows back into the mother liquor tank, then into thecirculation of the cooling cycle. A disadvantage of this process is thatthe method of grading to two fractions--the material discharged by thescrew and the material returned with the mother liquor--is difficult,because the grading has to be ensured by regulating the velocity of theflowing medium, at the same time the crystallization process is also afunction of the flow velocity. Owing to these difficulties, this processhas not gained acceptance.

OBJECTS OF THE INVENTION

The object of the invention is to provide a process for the extractionof solids from solution by crystallization, as a result of which highlypure crystals of graded grain size can be obtained.

SUMMARY OF THE INVENTION

The invention is based on the recognition that when grading of thecrystal slurry is carried out with continuous filtering, thencrystalline material of excellent quality and graded very keenlyaccording to grain size is obtained, the purity of which can beconsiderably increased with washing immediately after or during thefiltration; on the other hand seeding of the fresh solution with motherliquor containing a fraction of smaller grain size than the separatedone, can be ensured by dispensing with seeding of the solution withcrystal nodules and such crystal nodules may occur only eventually in avery small quantity during the process. According to a furthersignificant recognition, when the material is kept in constant motionalong the cooled surfaces during cooling and filtering, these surfaceswill be continually regenerated, the crystal slurry can be kept in themetastable zone, namely in the most favorable state in respect of thecrystallization, i.e. on the border of the transition to the unstablezone, thus the supersaturation of the mother phase does not take placein the vicinity of the cooling surface, crystallization of the coolingsurface is eliminated, and intensity of the crystal growth is maximal.

On the basis of this recognition, the process according to the inventioncalls for the mixture of the fresh solution and mother liquor of lowertemperature to form a mother phase in the metastable range, wherecrystallization begins, the mother phase being further cooled to such anextent as to remain in the metastable range, increasing the crystalgrowth. The crystals are then separated from the crystal slurry, andthis process is characterized by carrying out the crystal separationfrom the crystal slurry by grading the crystal slurry with continuousfiltration in such a way that the crystal slurry is separated only tothe fraction containing crystal grains of the required size, and to themother liquor containing crystals smaller than the former ones and theso-obtained mother liquor being continuously mixed with fresh solution;the fresh solution together with the crystal grains in the mother liquorsettles down by gravity, while the liquid mother phase of lower specificgravity than that of the fresh solution is discharged; formation of thecrystal slurry is intensified by cooling and simultaneous movement ofthe mother phase containing crystals separated from the mother liquorand the fresh solution, the crystal slurry being filtered and the aboveoperations are repeated at a continuous rate. According to a preferableimplementation method of the process the mother liquor is returned tothe fresh solution in a quantity exceeding 2 to 15 times the quantity ofthe discharged solution not containing solids.

The main feature of the apparatus for the continuous process is that itis provided with a storage tank for mixing of the fresh solution andmother liquor; pipeline and connected pump for delivery of the motherphase; treating device for the formation of crystal slurry from themother phase by cooling; device for filtering the crystal slurry;pipeline for feeding of the mother liquor containing crystals separatedby the filtration into the storage tank, whereby the tank, treatingdevice and filter device are interconnected in a closed system.

The advantages of the invention can be summed up as follows:

The invention ensures a graded product of excellent quality,distribution of the crystal grains is uniform, their size is large,shape convenient, compact. The impurity can be simply washed down fromthe crystals in the filter device. Space requirement for the apparatusgroup is small, its investment and running costs are economical. Inspite of the fact that the material handling during the process isgentle, there is no sedimentation or crystallization on the walls due tothe constant sliding movement of the material. As a result of theregulated cooling no large number of crystal nodules or large number ofsmall crystals occur, the crystals grow constantly in the treatingdevice, the temperature difference between the cooling surface andcrystal slurry is not excessive, thus the crystal nodules do not settleon the surface. The gentle movement prevents the large-size crystalsfrom fragmentation and the small-size crystals from sticking together.Seeding of the solution with crystal nodules is not necessary, since thecrystal slurry kept in suspension in the mother lye tank always containscrystals of small grain size in sufficient quantity to start off thecrystallization, and the fresh solution is fed continuously to thiscrystal slurry, i.e. the process is self-seeding. The heat transfercoefficient within the treating device is extremely favorable due to themixing. The grain quality (size, shape) suitable for the existingdemands depends on the ratio of the mother phase and mother liquor, aswell as on the r.p.m. of the treating body and on the angle of thetreating device and/or filter to the horizontal; their variation enablesthe control of the quality. The solvent leaving the equipment can berecovered by condensation of the vapors. The crystal size can beinfluenced also by taking the ratio of the grading crystalsuspension--mother liquor from the filter--and the solution from thegravitational sedimentator not containing solids to a high value, i.e.the ratio of recirculation is high. In this case for instance thecrystal grains may be large, but only a relatively small proportion isobtained from the process.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in detail on the basis of the sole FIGURE ofthe accompanying drawing showing a preferable embodiment of theapparatus used for the continuous process.

SPECIFIC DESCRIPTION

The apparatus has a storage tank 1, with a pipe 2 for admission of thefresh solution from the top, and a pipe 3 for feeding in the motherliquor, while the lower tapered part of tank 1--at the lowestpoint--ends in a pump 4, connected with a pipe 5 for delivery of themother phase. Pipe 5 contains a heat exchanger 6 provided with inletpipe stub 6a at the bottom and outlet pipe stub 6b at the top. A pipe 8runs concentrically in the vertical cylindrical tank 7 of the heatexchanger 6. Pipe 5 ends at hopper 9 to which the worm conveyor 10 isconnected. The cylindrical housing 11 of the worm conveyor serves as oneof the mechanical axes of rotation of the mixer device 12, coincidingwith the longitudinal horizontal geometrical axis x. The treating device12 has a first section I and second section II; the first one is formedby a truncated cone-shaped drum 13 and the latter one is formed by thepolygonal prismatic members 14, 15, 16 and 17 leading into each otherand rigidly connected to each other and to the drum 13. Drum 13 andmembers 14, 15, 16 and 17 together form a hollow body 18, rotatablealong the geometrical longitudinal axis x with the aid of drive 19 inthe direction of arrow w. Opening 21 eccentrically arranged in relationto the axis x is formed in the large end plate 20 of drum 13, the firstprismatic member 14 joining the opening 21 in such a way that its (notillustrated) geometrical longitudinal axis intersects the geometricallongitudinal axis x of the hollow body 18 outside the drum 13. Thegeometrical longitudinal axes of members 14, 15, 16 and 17 form azig-zag line with each other, and the axis of each member intersects thegeometrical longitudinal axis x outside the drum 13. The members 14, 15,16 and 17 together form a sawtooth shape. The last prismatic member 17ends in a ring rotating in a bearing 22 of fixed position. This latterone is fixed in the wall of a stationary box 23. The axis 24 of thehollow body 18 is carried in bearing on the other side wall of the box23; this axis is connected with the already mentioned drive 19. The wormconveyor 10 extends into the interior of drum 13, through its side plate32. Mixer 26 provided with blades 25 is arranged in drum 13, the bladesrunning along the inner surface of the tapered jacket with a smallspacing parallel with the generatrices of the surface. The mixer rotatescounter to the direction of rotation of the drum. Mixer 26 is rotatedfor instance from the shaft of worm conveyor 10 with drive 27. The wallsof both the drum 13 and prismatic members 14, 15, 16 and 17 are solid(not perforated) and the walls are duplicated separately, formingjackets to which a cooling medium can be supplied from pipeline 28, andthe heated cooling medium being discharged through pipe 29. Theduplicated, closed spaces are interconnected with pipes 31. Pipe 41emerges from the cover of box 23 leading into the heat exchanger 42.Otherwise the treating device 12 is described in detail in the Hungarianpatent application no. RI-699 (which corresponds to U.S. Ser. No.098,042 filed Nov. 28, 1979).

Pipe 33 extends downward from the lower part of box 23 leading into theworm conveyor 34. This latter one has a cylindrical housing 35, servingas the mechanical axis of rotation of the hollow body 37 of the filterdevice marked with reference number 36. Filter device 36 has a closedhousing 38 at the bottom of which troughs 39 and 40 are formed. Thehollow body 37 runs in the closed house 38 in longitudinal direction andit is provided with first section Ia and second section IIa. The firstsection Ia is formed by the truncated cone-shaped drum 43 through thesmaller end plate 44 by which the screw 34 is led into the interior ofthe drum, while the polygonal opening 46 is formed in the larger endplate 45. Opening 46 is eccentrically arranged in relation to thelongitudinal horizontal geometrical axis of rotation y of the hollowbody 37. Section IIa is formed by four rigidly interconnected polygonalprismatic members 47, 48, 49 and 50 leading into each other, thegeometrical longitudinal axis of which together form a zig-zag line,thus the members 47, 48, 49 and 50 appear in sawtooth shape. The firstmember 47 is rigidly connected to the eccentric opening 46 of drum 43 insuch a way that its geometrical longitudinal axis intersects thelongitudinal axis of the hollow body 37 outside the drum 43, likewise asthe geometrical longitudinal axes of members 48, 49 and 50. The walls ofthe drum and members are formed by the filter 51, or sieve (e.g.perforated plate, net, etc.). The last prismatic member 50--with a ring53 at its end--is rotatable in bearing 53a fixed to the side wall of thestationary box 52, this last member leading into the box 52 which isopen at the bottom, while pipe 54 emerges from the top. The washingdevice 55 extends through the last three members 48, 49 and 50, the pipe56 of the washing device 55 being fitted with spray heads 57. Pipe 56passes from the hollow body 37 through the box 52 and is connected to a(not illustrated) liquid source. The washing device 55 is in co-rotationwith the hollow body 37. For the sake of simplicity the drive of thehollow body is not shown. The section of the hollow body 37 containingthe washing device 55 is marked with reference letter A, the othersection formed by drum 43 and prismatic member 47 is marked with B.

The pipe 3 leading into tank 1 is branched off the trough 40 belowsection B. Pipe 58 leading into tank 59 is branched off the trough 39below section A.

A regulator device 60 is built into the pipe 2 outside the tank 1serving for admission of the fresh solution into the tank 1, the lowerend of said pipe extending below the liquid level v. Pipe 61 emergesfrom tank 1 at the height of the liquid level v, into which theregulator device 62 is built. Gas outlet pipe 63 emerges from tank 1 atthe top. Pipe 64 is branched off the pipe 5 before the heat exchanger 6,which is connected back to pipe 5 after the heat exchanger, but beforethe treating device 12.

The tank 7 of the heat exchanger 6 according to FIG. 1--depending on thedemand for heat extraction--may contain more than one pipe 8, andinstead of the illustrated uniflow cooling liquid, counterflowconnection is also possible.

Operation of the equipment according to FIG. 1 is as follows:

The fresh solution containing solids dissolved in warm, organic solventis delivered through pipe 2 below the liquid level v--in a quantityregulated with the device 60--into tank 1, and in addition, the motherliquor containing crystals--and possibly small quantity of crystalnodules as well--obtained in the filter equipment 36 as a result of thecontinuous crystallization process, is also returned to the tank 1,through pipe 3. The crystal grains of higher specific gravity settlefrom the mother liquor at the bottom of tank 1, while the mother liquorof lower specific gravity not containing crystal grains isdischarged--in a quantity regulated with the shut-off device 62--fromtank 1 at the top through pipe 61. The mother phase accumulated at thebottom of tank 1 is pumped into pipe 5 with screw pump 4, then flowsthrough heat exchanger 6 and subjected to preliminary cooling. Thecooling medium enters the heat exchanger through pipe stub 6a at thebottom, withdraws heat from the mother phase and leaves through pipestub 6b, while the mother phase flowing upwards in pipe 8 is subjectedto preliminary cooling to the temperature required by the existingtechnological operation, then pumped into the hopper 9. (If precoolingis not necessary the mother phase passes into the hopper 9 throughbranch pipe 64 by-passing the heat exchanger 6.)

The mother phase is fed with worm conveyor 10 into the rotary treatingdevice 12, the interior of which is cooled with cold liquid through thejacketed walls. The mother phase in drum 13--i.e. in the first sectionI--is mixed with mixer 26 during the process of cooling, and the drumdischarges the already partially crystallized material into theprismatic member 14 of the second section II.

Due to the rotation of members 14, 15, 16 and 17 which form a sawtoothshape in the second section II, the cooled, supersaturatedsolution--crystal slurry--dividing continuously during its progress, isforced to a forward-backward movement, sliding back repeatedly on thesmooth internal surface (which is preferably of smooth,adhesion-resistant material, e.g. teflon), so that consequently thedwelling period of the crystal slurry in the treating device 12 isextended, in other words the specific internal surface of the prismaticmembers 14, 15, 16 and 17 wetted with liquid is increased. As a resultof the constantly dividing-sliding movement and operation of the mixerdevice 26, the internal surfaces of the hollow body 18 are constantlyregenerated, the solid content of the crystal slurry does not stick tothe surface, supersaturation of the mother phase can not occur in thevicinity of the cooling surface, the mother phase remains in themetastable zone, thus crystallization of the cooling surface iseliminated. In addition these material movements are very gentle,whereby not only the cohesion of the crystals, but their fragmentationis also excluded.

The cooled crystal slurry containing crystals of various grain sizepasses from the last prismatic member 17 of the treating device 12 intothe box 23, then into the worm conveyor 34. The vapors released from themother liquor pass from the upper part of box 23 through pipe 41 intothe heat exchanger 42 (condenser, after-cooler) and are cooled down andcondensed and thus will be collectible in the form of a liquid(solvent).

The crystal slurry is delivered with feeder screw 34 into the rotaryhollow body 37 of the filter device 36, the hollow body beingaccommodated in the closed housing 38. The material first passes intodrum 43--into the first section Ia of the filter device--where thelarge-size crystal grains are filtered out of the crystal slurry. Themass of material consisting of the filtered but still wet crystal grainsis delivered by drum 43 into the first prismatic member 47 of the secondsection IIa and the material passes through the rotary sawtoothshape-forming members 47-48, 49, 50 of the second section IIa in such away that it is forced into a forward-backward movement, sliding backrepeatedly and thereby continuously regenerating the filter 51 or sievesurfaces. The dwelling period of the material and the specific surfaceof the sieve surfaces are considerably increased. Here the movement isalso very gentle, the crystals do not become fragmentary, and nocoherence or sedimentation occurs on the surfaces.

There is no washing device in the range B of the second section IIa,thus the mother liquor (crystal suspension) passes off and accumulatesin trough 40, then flows back continuously through pipe 3 into tank 1.The washing device 55 extends in the range A of the second sectionIIa,--i.e. in members 48, 49 and 50--through which washing liquid, e.g.organic solvent can be delivered to the crystalline mass of materialwhich is in sliding motion. The impure washing liquid flowing throughthe sieves accumulates in tub 39, and from here it passes through pipes58 into tank 59. The vapor ascending from the crystal slurry leaves theclosed housing 38 through pipe 54. The separated, large crystals ofuniform grain size pass continuously downward through pipe 52, and maybe dried.

It is noted that the Hungarian patent application no. RI-699 gives amore detailed description of the filter device 36 (see copendingapplication Ser. No. 098,042).

In the following the invention is described in detail by means of anexample as follows:

The object is crystallization of phenylbutazone. Concentration of thesolution: 58 g phenylbutazone/liter of solvent at a temperature of 50°C. Composition of the solvent: 80% ethyl alcohol, 20% water. Feedingtemperature of the fresh solution: 50° C. Recirculation ratio of themother liquor: Q=10. No precooling is applied. In both sections of thetreating device 17° C. temperature is maintained. Temperatures of theoutgoing crystal slurry: 22° C., yield 98%, volume weight of the crystalaggregation: 25-32 g/100 ml. The crystal is compact, large sized, welltreatable during tabletting.

What we claim is:
 1. In a process for the extraction of dissolved solidmaterial from a solution by crystallization in which a mother phase in ametastable range in which crystallization begins is formed, said motherphase is cooled to increase crystal growth and form a crystal slurry,and crystals are separated from said slurry, the improvement whichcomprises the steps of:(a) separating said crystals from said crystalslurry by continuous filtration of said slurry to form a fractioncontaining a required grain size of crystals and a fraction of motherliquor containing crystals of a grain size less than said required size;(b) continuously mixing the mother liquor from step (a) with freshsolution containing dissolved solids; (c) allowing the crystals from themother liquor and the fresh solution to settle and form said motherphase and a further mother liquor free from crystals, and dischargingsaid further mother liquor; (d) cooling and simultaneously moving themother phase formed in step (c) to increase crystallization thereof andform said crystal slurry, the cooling and simultaneous moving of saidmother phase formed in step (c) being effected by passing it alongcooled surfaces from one end to another of an elongated body whileswinging said body around an axis; and (e) feeding the crystal slurryformed in step (d) to filtration in step (a).
 2. The process defined inclaim 1 wherein the mother liquor mixed with the fresh solution in step(b) is of a quantity 2 to 15 times greater than that of the motherliquor withdrawn in step (c).
 3. The process defined in claim 2 whereinthe crystals separated in step (a) are washed with a solvent-typeliquid.
 4. In a process for the production of crystals of a particulargrain size from a solute in a solution by crystallization in which amother phase in a metastable range in which crystallization begins isformed, said mother phase is cooled to increase crystal growth and forma crystal slurry, and crystals are separated from said slurry, theimprovement which comprises the steps of:(a) separating said crystals ofsaid particular grain size from said crystal slurry in a continuousfiltration by passing said crystal slurry through an elongated filterbody having angularly adjoining linear segments each of which is formedby a sieve of an opening size adapted to retain crystals of saidparticular grain size and to pass a mother liquor containing crystals ofa grain size less than said particular grain size, and rotating saidfilter body about an axis offset from the axis of said filter bodywhereby the slurry moves back and forth in each segment as it progressesthrough said body; (b) collecting from an end of said filter bodycrystals of said particular grain size; (c) collecting the mother liquorcontaining crystals of a grain size less than said particular grain sizeand passing through said filter body and feeding the collected motherliquor to an upright storage tank; (d) adding to the collected motherliquor in said storage tank a fresh solution containing said solute toform said mother phase in said metastable range and permitting saidmother phase to settle in said storage tank; (e) withdrawing said motherphase from the bottom of said storage tank and inducing furthercrystallization therein by passing said mother phase through anelongated crystallizer body having a plurality of angularly adjoiningprismatic linear segments defined by respective cooling surfaces,rotating said crystallizer body about an axis offset from thelongitudinal axis thereof, and cooling the walls of said crystallizerbody whereby said crystal slurry is formed in said crystallizer body;and (f) feeding the crystal slurry formed in step (e) to said filterbody of step (a).
 5. The improvement defined in claim 4, furthercomprising the steps of:(g) decanting mother liquor free from crystalsfrom an upper portion of said storage tank; and (h) precooling at leasta portion of said mother phase before it is introduced into saidcrystallizer body in step (e).
 6. The improvement defined in claim 5,further comprising the step of:(i) mixing a portion of the precooledmother phase of step (h) with another portion of said mother phase whichhas not been precooled in step (h), prior to introducing the mixture ofsaid mother phases into the crystallizer body of step (e).